The invention relates to methods of making superconductors having epitaxial layers.
Superconductors are used in a variety of applications. Often, the mechanical integrity of a superconductor can be enhanced by forming a multilayer article that includes a layer of superconductor material and a substrate layer, but the use of a substrate can present certain complications.
Chemical species within the substrate may be able to diffuse into the layer of superconductor material, potentially resulting in a loss of superconductivity. Moreover, the coefficients of thermal expansion as well as the crystallographic spacing and orientation of the substrate and the superconductor layer can be different, causing the article to peel apart during use.
To minimize these complications, a buffer layer can be disposed between the substrate and the superconductor layer. The buffer layer should reduce diffusion of chemical species between the substrate and the superconductor layer, and the buffer layer should have a thermal coefficient of expansion that is about the same as both the substrate and the superconductor layer. In addition, the buffer layer should provide a good crystallographic match between the substrate and the superconductor.
One approach to controlling the crystallographic properties of a layer has been to use epitaxy. An epitaxial layer is a layer of material that is grown on the surface of a substrate such that the crystallographic orientation of the layer of material is determined by the lattice structure of the substrate. By epitaxy is also meant materials with ordered surfaces whether formed by conventional epitaxy or graphoepitaxy. Epitaxial layers have been grown using physical vapor deposition (PVD), chemical vapor deposition (CVD) and sputtering techniques.
Typically, PVD involves the evaporation of a solid material and transfer of the vapor to the substrate surface in a diffuse gas beam in which only a small portion of the total amount of evaporated solid may reach the substrate surface. Thus, the material usage efficiencies obtained with PVD can be low. In addition, PVD is usually performed at a chamber pressure of at most about 1.times.10.sup.-4 torr, so the flux of evaporated solid at the substrate surface can be small, resulting in low epitaxial layer growth rates.
In CVD, one or more reactant gases within the chamber adsorb to the substrate surface and react to form the epitaxial layer with product gases desorbing from the substrate surface. Generally, the reactant gases reach the substrate surface by convection and diffusion, so the material usage efficiencies can be low. Furthermore, CVD is typically conducted at a chamber pressure of at least about 0.1 torr, and, to grow epitaxial layers at these elevated pressures, relatively high substrate temperatures are usually used. Thus, the selection of substrate materials used in CVD can be limited.
Sputtering methods of growing epitaxial layers can also be limited by the aforementioned considerations.
When using PVD, CVD or a sputtering technique, the quality of the epitaxial layer can depend upon the chemical nature of the substrate surface during layer growth. For example, contaminants present at the substrate surface can interfere with epitaxial layer growth. In addition, native oxides present at the substrate surface can help or hinder epitaxial layer growth. Further, PVD, CVD and sputtering methods can be ineffective at providing control of the chemical nature of the substrate surface during layer growth, so the epitaxial layers formed by these techniques can be of poor quality.